Method of producing plastic composites filled with cellulose or lignocellulosic materials

ABSTRACT

A method to produce composites based on cellulose or lignocellulosic materials and plastics according to which method the cellulose or lignocellulose material is subjected to a pre-hydrolytic or other chemically degrading treatment prior to or during the compounding or processing step whereby a comminution and improved dispersion of the cellulose or lignocellulose material in the plastic phase is achieved.

The focus on energy has had a significant impact on the growth ofinterest for fillers and extenders for plastic materials. It is knownthat such additives may influence the property profile of the finalproduct or, in certain cases, act as pure extenders allowing a moreefficient use of the plastic component. There are also instances wherethe filler may have a marked reinforcing effect.

Among the many types of fillers and similar additives, cellulose andvarious kinds of lignocellulosic materials take an important place.Especially interesting is the increasing use of such fillers inthermoplastics. Cellulose fibre, cellulose flour, wood flour, and nutshell flour are a few examples of additives to be discussed in thepresent context.

When the lignocellulosic or cellulose based material is used as powderor flour, it has to be ground prior to the compounding step. When usedas fibres, such materials experience a certain grinding action in theplastics processing machinery, but, this effect is, however, of littlepractical significance. Fibrous additives/fillers of the above mentionedtype are normally expected to possess a certain reinforcing effectreminiscent of that obtained with glass fibres or other similarreinforcement.

When using additives (fillers, reinforcements) of the present typedifficulties are normally encountered in achieving a sufficient degreeof dispersion during the compounding and processing steps, especiallywhen the additive is fibrous. An acceptable dispersion is normallyobtained only with finely ground material. When trying to achieve anacceptable dispersion with fibrous matter, the mixture has to besubjected to such an intense and repeated kneading action during thecompounding stage, that the time and energy consumed in that stage makesuch an approach prohibitive.

Such an energy consuming comminution of the cellulosic orlignocellulosic additive in the compounding step, for instance byrepeated treatment in a kneader, fulfills no proper technical task, asit could have been done easier by grinding the filler prior tocompounding. On the other hand, it is believed that an excessivecomminution of the fibrous filler should be avoided, on the assumptionthat the reinforcing action would be lost.

When investigating the effect of cellulosic and lignocellulosic fillerson the property profile of thermoplastic composites, we havesurprisingly found that the particle size of the filler/reinforcingagent plays a minor role only, implying that the reinforcing effect ofsuch fillers, when present in fibrous form, is not more pronounced thanthat of finely divided particulate matter. This finding has an importantpractical implication, as it justifies the use of finely divided fillerswhich are easy to disperse in the plastic matrix.

The aim of the present invention is the use of easily disintegrablepre-hydrolysed cellulosic or lignocellulosic material as additive toplastics, preferably thermoplastics. It is known that a pre-hydrolysisof such materials results in a substantial embrittlement. Thisembrittlement is utilized in the present invention in the compoundingstage, where the pre-hydrolyzed material is added to the plasticcomponent without prior grinding or other comminution. The comminutionto the final particle size takes place in the compounding machine or,directly, in the processing machine as an effect of the shear forcesprevailing in such machines (kneaders, extruders, injection mouldingmachines, film blowing equipment etc.). The pre-hydrolysis of theadditive thus eliminates the necessity of the normal time and energyconsuming mechanical comminution prior to the compounding step.

It is understood that the additive has a suitable particle size beforebeing subjected to the hydrolytic embrittlement. To illustrate thispoint, we may refer to our experiments where normal wood flour, particlesize 0.1-0.5 mm, or about 2×2 cm large pieces of cellulose pulp sheetshave undergone hydrolytic degradation, whereafter they were directly fedinto the compounding equipment (Buss Ko-Kneter type PR46, diameter 46 mmL/D 11) producing a well dispersed compound which was easily processedon an injection moulding machine. Similar experiments with the additionof same materials which have not undergone any hydrolysis produced acomposite with a poor dispersion which, especially in the case offibrous cellulose, was entirely unacceptable.

The pre-hydrolysis as such is a well-known method, for instance in theproduction of so called microcrystalline cellulose. Although literaturedata relating to the embrittlement by hydrolytic agents only refer topure cellulose, we found that the embrittlement which is a necessaryelement of the present invention can be achieved also withlignocellulosic materials such as wood flour of varying chemicalcomposition, coarser wood particles, straw etc.

It is understood that the pre-hydrolysis of the filler component iscarried out by the action of suitable chemical agents. Normally,inorganic acids in dilute aqueous solutions fulfill this task properly,the concentration, and time and temperature of treatment depending onthe chemical nature of the filler. Also organic acids as, for instance,formic or oxalic acid, various aromatic sulphonic acids may be used.Some acidic substances, as hydrogen chloride or hydrogen fluoride, maybring about the hydrolyzing action in gaseous form. This has animportant practical advantage in that the hydrolytic embrittlement maybe carried out without using a wet method, thus eliminating the dryingstep. The gaseous agent, such as hydrogen chloride or hydrogen fluoride,can be easily driven off by air or other inert gas, at normal orelevated temperatures, or by simple heating. When required, remainingacidity may be eliminated by suitable neutralization. This applies toall types of acidic hydrolysis. In certain cases, the hydrolizingsubstance may be left in the filler without subsequent neutralization.

The embrittlement to be carried out before using the filler as componentin the plastic matrix may also be achieved by other substances known toproduce a hydrolytic attack on the substances considered here. Asexamples of such substances may be mentioned alkaline solutions orgases. Also in this case, the substance remaining in the filler afterthe treatment may be neutralized when necessary.

The effects intended in the present invention can also be achieved bytreating the filler with acidic salts at suitable conditions oftemperature and time of treatment, an example of such a substance beinghydrogen potassium sulphate.

Also substances known to split off acidic components at elevatedtemperature or after having reacted with the cellulosic orlignocellulosic material may be utilized in the present context. Anexample is cyanuric chloride which, upon reacting with the hydroxylgroups of cellulose or lignin, splits off hydrogen chloride whichproduces the embrittling attack on the filler.

The list of substances which may be used in the sense of the presentinvention also contains metallic salts known to produce degradation ofthe fillers in question in the presence of oxygen. Also ozone treatmenthas a similar effect.

It is understood that not only the types of substances exemplified abovemay be used according to the present invention, but also suitablemixtures of such substances, as well as combinations of the variousmethods of treatment.

It is further understood that the liquid vehicles used to carry out theheterogeneous pre-hydrolysis may be water or any other liquid.Non-aqueous solvents as, for instance, liquid sulphur dioxide, liquidammonia or organic solvents, have often the advantage of needing lessthermal energy when the product is to be dried.

It is to be stressed that the pre-hydrolyzed material after washing,neutralization and drying may be directly used as filler which attainsthe final degree of comminution to a finely divided matter in theprocessing equipment. In certain cases, the mixture of filler andplastic may be fed directly into the processing machinery without priorhomogenisation in the compounding stage.

When the product to be processed so allows, the hydrolytic agent may beleft in the filler. This in, for instance, the case with certain organicacid as formic or oxalic acid. It is also possible to arrange theprocess so that the remainders of the hydrolyzing agents may be drivenoff in the venting arrangement of the compounding or processingmachinery (injection moulding machines, extruders etc. equipped withvented screws).

The comminution in the compounding and/or processing machinery is,obviously, carried out at a substantially lower energy consumption thatif it were necessary to perform the comminution without the beneficialaction of the hydrolytic attack. In this sense, cellulosic orlignocellulosic fillers pre-hydrolyzed according to the presentprocedure may be termed self-comminuting.

EXAMPLES of hydrolytic pre-treatment in order to attain a high degree ofself-dispersion of cellulosic or lignocellulosic fillers in normalprocessing of filled thermoplastics

Bleached spruce sulphate pulp was treated with 5% aqueous hydrochloricacid at room temperature for 10 hours. After washing and drying, thematerial could be easily converted to a fine powder by gentle mechanicalaction. Similar results were obtained with 5% or 10% sulphuric acid. Thetime of treatment could be reduced down to minutes by increasing thetemperature to 70° C.

Also unbleached spruce sulphate and sulphite pulp was easily convertedto fine powder by the above procedure.

The same was true of wood flour of different origin, both from conifersand deciduous species. The fibrous structure remaining in normallyavailable wood flour disappeared entirely after the hydrolytictreatment.

Easily powderized matter was also produced by treating bleached andunbleached pulps and wood flour of different origin by treatment withdilute solutions of cyanuric chloride in ethyl alcohol, drying to afinal content of cyanuric chloride of 2% in the filler and heating to120° C.

In order to test the dispersability of the pre-hydrolyzed matter duringprocessing of the corresponding composites, injection mouldingexperiments were performed with high density polyethylene as the matrixmaterial. In these experiments, a mixture of 60% high densitypolyethylene (Unifos DMDS 7006) and 40% pre-hydrolyzed (5% hydrochloricacid) bleached spruce sulphate pulp was compounded in a Buss Ko-Kneter(type PR46, screw diameter D=46 mm, L/D 11) and the homogenized mixtureinjection moulded using a conventional machine (Arburg 22 IE/170 R).

It was found that the cellulosic component exhibited a high degree ofdispersion, the average particle size being 30 μm. It is to be remarkedthat the pre-hydrolyzed pulp was fed with the HDPE component into thecompounding machine in the form of 2×2 cm² large pieces (thickness 2mm). Tensile testing of the mouldings (tensile testing bars, DIN 53455,dimensions area 10×3,5 mm², length 150 mm) gave the following result:

tensile strength: 21 MPa

breaking elongation: 6.7%

tensile modulus: 2700 MPa

impact strength: 20 kJ/m² (charpy, unnotched)

Experiments with untreated pulp gave lower values of the aboveparameters, especially the breaking elongation which was about 1% only.Mainly, however, the dispersion was entirely insufficient.

Measurements were also done on corresponding compounds containingcommercial microcrystalline cellulose (Mikrocell, Avicel PH 102, averagesize 50 μm, and Avicel PH 105, average size 20 μm). In this case themodulus was around 1800 MPa, the tensile strength around 13 MPa and thebreaking elongation 2-3%. The impact strength was ca. 18-19 kJ/m². Allresults relate to samples conditioned at 50% rel. humidity and 23° C.The strain rate of the testing machine (Instron, type 1193) was 10mm/min.

These findings support the ideas underlying the present invention inthat they show that it is not necessary to retain the fibrous structureof cellulosic fillers in order to obtain an reinforcing effect in thecomposite. On the contrary, due to poor dispersion of such fibrousfillers, they have a detrimental effect on the property profile. Anothereffect clearly shown by the above findings is the feasibility of easilyattaining a high degree of self-comminution of pre-hydrolyzed cellulosicfillers in normal processing equipment.

To supplement the above findings experiments were also carried out withinjection moulding of HDPE filled with 40% of spruce wood meal. A slightimprovement in the mechanical property profile was noted when usingprehydrolyzed meal. The main finding, however, was a substantialreduction in particle size and a better dispersion.

An important consequence of the self-comminuting effect discussed aboveis the possibility to incorporate larger amounts of the filler into theplastic matrix. In a series of experiments 70% of prehydrolyzed bleachedsulphate pulp was incorporated into polypropylene (ICI GYM 121), to becompared with 50% of the untreated pulp, and 60% of the correspondingcellulose meal having an average particle size of 70 μm.

In the light of what has been said above it thus appears clear that thepresent invention solves an important problem in connection with usingcellulosic or lignocellulosic fillers in plastics, in the first handthermoplastics, that is to say the problem with achieving a satisfactorydispersion of the filler particles in the plastic matrix. At the sametime, an improvement of the mechanical property profile is achieved.

A particularly interesting instance of the difficulties associated withthe improper dispersion of the filler is encountered in the recovery ofplastic waste containing paper. Such waste is recovered from municipalrefuse; it is also found in plants for the manufacture of laminatedpackaging materials based on, for instance, polyethylene and paper. Insuch cases, the present method provides an efficient means in convertingsuch waste into well dispersed composites.

We claim:
 1. A method of producing plastics composites containingcellulose or lignocellulose material in a plastics matrix, said methodcomprising the successive steps of:(1) subjecting the cellulose orlignocellulose material to pre-hydrolytic chemical degradation, andthereafter (2) incorporating the thus degraded cellulosic orlignocellulosic material into the plastics matrix thereby facilitatingcomminution and improved dispersion of the cellulosic or lignocellulosicmaterials in the plastic phase.
 2. The method of claim 1 in which theplastics composite contains up to 40% by weight of the cellulose orlignocellulose material.
 3. The method of claim 1 in which a masterbatchconcentrate for plastics containing up to 70% by weight of the celluloseor lignocellulose material is prepared.
 4. A method of producing athermoplastic composite in which cellulose of lignocellulose materialsare dispersed, said method comprising the successive steps of:(1)subjecting a cellulose or lignocellulose material to a pre-hydrolytictreatment to produce an embrittled easily disintegratable,self-comminuting material, and thereafter (2) incorporating thethus-treated material into the thermoplastic and subjecting theresulting mixture to shear forces thereby comminuting the treatedcellulosic or lignocellulosic material and distributing same throughoutthe thermoplastic.
 5. The method of claim 4 in which the cellulose orlignocellulose material is not subjected to grinding or comminutionprior to the pre-hydrolytic treatment of step (1).
 6. The method ofclaim 5 in which the thermoplastic composite contains up to 40% byweight of the pre-hydrolytic cellulose or lignocellulose incorporatedtherein.
 7. The method of claim 5 in which a masterbatch concentrate isproduced which contains up to 70% by weight of the pre-hydrolyticcellulose or lignocellulose incorporated therein.